Method of treating titanium-containing structures

ABSTRACT

A method is disclosed for the removal of metallic iron inclusions from the surfaces of chemical process equipment, particularly electrolytic cells, prepared from titanium. This method involves the treating of titanium alloy surface with an acid solution to remove metallic iron inclusions from those surfaces of the titanium that are subject to attack by halides.

waited States Patent DuBois METHOD OF TREATING TITANIUM-CONTAINING STRUCTURES Donald W. DuBois, Corpus Christi, Tex.

Assignee: PPG Industries, Inc., Pittsburgh, Pa.

Filed: Mar. 31, 1972 Appl. No.: 239,991

Inventor:

References Cited UNITED STATES PATENTS 10/1944 Curtis 156/7 X 1 Sept. 17, 1974 2,679,474 5/1954 Pajes 156/7 X Primary ExaminerWilliam A. Powell Attorney, Agent, or FirmRichard M. Goldman [57] ABSTRACT A method is disclosed for the removal of metallic iron inclusions from the surfaces of chemical process equipment, particularly electrolytic cells, prepared from titanium. This method involves the treating of titanium alloy surface with an acid solution to remove metallic iron inclusions from those surfaces of the titanium that are subject to attack by halides.

12 Claims, N0 Drawings METHOD or TREATING TITANIUM-CONTAINING STRUCTURES BACKGROUND OF THE INVENTION In the construction of chemical process equipment for use in environments where free halogens are present, such as electrolytic cells for the production of chlorine by the electrolysis of brines, titanium is frequently used as a material of construction. This is because of the tendency of titanium to force a corrosionresistant film under oxidizing conditions. However, such titanium reaction vessels are particularly subject to attack under conditions of oxygen depletion at crevices such as welds, joints, laps, filletts, and the like. This type of corrosion is characterized as crevice corrosion.

Attempts to prevent crevice corrosion or to substantially reduce the effects of it have typically focused on various organic coatings on the titanium, inorganic coatings on the titanium, and the use of titanium alloys. One particularly satisfactory titanium alloy is an alloy of titanium and nickel. Such a titanium alloy and its use in halide solutions is disclosed in US. Pat. No. 3,469,975 to Bertea et al. for Method of Handling Crevice Corrosion-Inducing Halide Solutions. The alloy disclosed by Bertea et al. is a titanium alloy containing up to about 5 per cent nickel, at least about 0.3 per cent cobalt, and about 2.0 per cent molybdenum. Further disclosed therein is the corrosion-resisting effect of small additions of cobalt and molybdenum to titanium. While such titanium alloys exhibit marked crevice-corrosion properties when tested as coupons in heated brine, it has been found that after various types of mechanical working and fabricating operations, their susceptibility to crevice corrosion suffers a marked increase.

SUMMARY OF THE INVENTION It has now surprisingly been found that the presence of unalloyed surface iron, even in amounts as low as 5 parts per million, results in a marked increase in susceptibility to crevice corrosion of titanium and titanium alloys. So sensitive to the presence of iron is the alloy, that iron inclusion sufficient to materially increase susceptibility to crevice corrosion will be introduced to the alloy during mechanical working. Such inclusions are normally the result of various metal working and fabricating processes quite apt to be used.

It has further been found that the crevice corrosionresistant properties of such worked and fabricated titanium and titanium alloys may be maintained substantially undiminished if, subsequent to the working and fabricating processes, particular care is taken to re move metallic iron inclusions therefrom.

According to this invention, metallic iron inclusions are removed by treatment of the worked or fabricated surface with an aqueous liquid composition containing two acids, one of which is an oxidizing acid and the second of which is capable of reacting with iron to form soluble iron salts.

Best results are obtained if the treatment of the worked or fabricated titanium or titanium alloy surface is continued until the amount of iron inclusions remaining in the surface is low enough that the susceptibility of the article to crevice corrosion is substantially reduced. Such a concentration of metallic iron inclusions is typically shown by the negative response to the surface of the article to standard tests for metallic iron.

DETAILED DESCRIPTION OF THE INVENTION According to this invention, titanium or titanium alloy structures subject to corrosion in oxygen deficient, halogen-containing environments may be rendered resistant to crevice corrosion by the removal of metallic iron inclusions from the titanium.

Titanium structures are preferred for use in the processing of halides and halogens. Such structures include apparatus for the desalinization of brines and brackish water, electrolytic cells for the production of chlorate, chlorine, and other halides, and various other chemical processing equipment. Titanium and its alloys are preferred materials of construction because of their tendency to form a corrosion-resisting film under oxidizing conditions. This film protects the underlining titanium material from further corrosive or oxidative attack. However, titanium structures are attacked at laps, filletts, crevices, compression fittings, and the like. They are also attacked under gaskets at edges of compression fittings, joints, laminations, and the like. This phenomena is referred to as crevice corrosion.

According to US. Pat. No. 3,469,975 to Bertea et al. for Method of Handling Crevice Corrosion-Inducing Halide Solutions, it is known that small amounts of alloying materials significantly increase the resistance of titanium structures to crevice corrosion. Such alloying materials include nickel, cobalt, and molybdenum. In the crevice corrosion-resistant alloys of Bertea et al., the quantity of nickel present is from about 0.1 per cent to about 5 per cent; the amount of cobalt present is from about 0.3 per cent to about 5 per cent; and the amount of molybdenum present is in the range of about 2 per cent. Alloyed iron, up to about 0.1 weight per cent may also be present in the alloy. Additionally, alloys of titanium with the precious metals, such as 0.2 per cent palladium alloy of titanium, have been found to be remarkedly resistant to crevice corrosion. Typical crevice corrosion-resistant alloys useful in the practice of this invention also include titanium alloys containing nickel, cobalt, molybdenum, niobium, aluminum, and tantalum. These alloying elements may be present singly as in Ti-Ni, Ti-Co, and Ti-Mo, or in combinations as in Ti-Al-Nb-Ta-Mo, and Ti-Al-Nb-Mo. In such alloys useful with the process of this invention, titanium is the major constituent, being or or more weight per cent of the alloy. Whenever titanium is referred to herein it will be understood to include alloys having titanium as a major constituent. It has been found that such materials as described hereinabove are particularly resistant to crevice corrosion when tested in the form of coupons where the crevices are formed by laboratory compressive means. However, these same materials suffer a significantly reduced reistance to crevice corrosion after fabrication into chemical processing apparatus.

It has now been found that the reduced resistance to crevice corrosion of normally crevice corrosionresistant titanium and titanium alloys is caused by the presence of any metallic iron inclusions in the alloy sufficient to cause pitting. This is generally about 5 parts per million of unalloyed, metallic iron. Such metallic iron inclusions are introduced during the manufacturing and fabricating steps. They may be introduced during the rolling, bending, forging, forming, and shaping of the titanium. Alternatively, the metallic iron inclusions present in the alloy may be introduced during the fabrication of the chemical apparatus itself, such as by clamping, machining, cutting, welding, and the like.

According to this invention, these minute inclusions of metallic iron are removed from the fabricated apparatus and the apparatus is thereby rendered more resistant to crevice corrosion. This is accomplished by treating the titanium or titanium alloy, particularly in those spaces and volumes subject to crevice corrosion, with an aqueous liquid composition of a strong oxidizing acid capable of forming insoluble titanium oxides and a second acid capable of forming soluble iron salts. Preferably, the acid treatment is continued until the concentration of metallic iron in the alloy surface is reduced below that level at which crevice corrosion is initiated. This is usually below about 5 parts per million on the surface and, for practical purposes, below the level below which standard wet chemical analytical tests for iron are negative.

The aqueous liquid composition used in the practice of this invention contains, as its chemically active ingredients, two acids. The first'acid is an oxidizing acid. By oxidizing acid is meant an acid that contains oxygen and that is capable of reacting with titanium to form an insoluble oxide surface on the titanium. Nitric acid is the preferred oxidizing acid. Perchloric acid and chromic acid may also be used. The second acid is an acid capable of reacting with iron to form soluble salts of iron. Suitable acids include the halo-acids, hydrofluoric acid, hydrochloric acid, and hydrobromic acid. Hydrochloric acid is the preferred halo-acid. Hydrofluoric acid may also be used in the practice of this invention, although care must be taken to stop the treatment of the surface before the hydrofluoric acid begins to solubilize the titanium. Hydrobromic acid may also be used in the practice of this invention although the higher cost of hydrobromic acid may render it less attractive than hydrochloric acid. Suitable acids also include strong organic acids capable of giving up hydronium ions, such as the tri-halogenated acetic acids, trichloroacetic acid and trifluoroacetic acid. Satisfactory results are also obtained with sulfuric acid.

The liquid composition used in the practice of this invention typically contains from about 5 to about 20 volume per cent of the oxidizing acid and, preferably, from about 7% to about 15 volume per cent of the total liquid composition. When volume per cents are referred to herein, such volume per cents are based on the volumes of the original reagents prior to mixing and do not include mixing effects. When the oxidizing acid is nitric acid, the concentration of nitric acid is preferably from about 7 /2 to about 15 volume per cent.

The concentration of the second acid is preferably from about 10 to about 40 volume per cent. When the second acid is hydrochloric acid, good results are obtained at from about to about 40 volume per cent of hydrochloric acid. Best results are obtained at about to about volume per cent hydrochloric acid. When the second acid is hydrofluoric acid, particularly satisfactory results are yielded in the range of from about 5 to about 10 volume per cent hydrofluoric acid.

In a preferred exemplification of this invention, nitric acid is the oxygen-containing acid, and hydrochloric acid is the second acid. According to this exemplification, liquid compositions having particularly satisfactory iron removal properties are provided in the range of from about 5 to about 15 volume per cent nitric acid and from about 15 to about 40 volume per cent hydrochloric acid. The preferred liquid composition of this exemplification contains from about 7 to about 15 volume per cent and preferably about 10 volume per cent nitric acid, and from about 20 to about 40 volume per cent hydrochloric acid, and preferably about 30 volume per cent hydrochloric acid.

Lower concentrations of the second acid, for example, less than about 10 volume per cent hydrochloric acid or lower than about 1 volume per cent hydrofluoric acid may be used to remove iron inclusions from titanium materials. However, such low concentrations of the second acid result in unnecessary long periods of treatment, for example, in excess of about 30 to about 45 minutes. Similarly, particularly high concentrations of the second acid, for example, liquid compositions of 40 volume per cent hydrochloric acid and 10 volume per cent nitric acid also require in excess of 45 minutes at 25C. to 30C. to remove substantially all of the iron. When the second acid is hydrofluoric acid, liquid compositions containing, for example, 10 volume per cent nitric acid and in excess of about 10 volume per cent nitric acid and in excess of about 10 volume per cent hydrofluoric acid, result in solubilization of the titanium.

The temperature of the liquid composition is such as to keep it a liquid; that is, between the freezing temperature and reflux temperature. Particularly satisfactory results are obtained at temperatures of from about 5C. to about 50C. Temperatures above about 50C. do not result in any significantly increased rate of removal of iron inclusions or in decreased treatment time. Furthermore, such temperatures give rise to problems related with soiubilization of the titanium. Accordingly, there is no incentive to go to temperatures above about 50C., although such temperatures are included within the scope of this invention. Temperatures below about 5C. result in a significantly increased times for the removal of the iron inclusions and, therefore, no incentive exists to go to temperatures below about 5C. although such temperatures are also included within the scope of this invention.

The time of treatment varies from about 5 minutes or less up to about 45 minutes or even longer. As described hereinabove, the time of treatment is a function of the concentrations and proportions of acids in the liquid composition and the temperature of the liquid composition. For example, at high temperatures the time necessary to obtain substantially complete removal of iron as determined by standard colorimetric tests is less than the time necessary to obtain the same degree of iron removal at lower temperatures. Similarly, in a liquid composition containing 10 volume per cent nitric acid and 20 volume per cent hydrochloric acid, the time required to obtain substantially complete iron removal is less than the time required to obtain an equivalent degree of iron removal with liquid compositions containing significantly more or significantly less hydrochloric acid.

In a further exemplification of this invention, reduced treatment times may be obtained by abrading the titanium alloy surface prior to treatment with the acid compositions. The titanium alloy surface of the apparatus may be abraded by any of the methods normally used for the abrasive cleaning of metal surfaces,

for example, a power-driven wire wheel may be used to remove surface inclusions of metallic iron. In the case of a carbide grinding wheel or an iron or steel or stainless steel wheel, or steel wool abrasives, the amount of iron inclusions deposited by the abrasive will generally be significantly less than the amount of iron removed by the abrasive will be more readily removed by the liquid composition than the iron inclusions deposited during the manufacturing process. Alternatively, sandpaper or sand blasting may be used as a suitable abrasive method for the preliminary removal of iron inclusion prior to the use of the acid composition.

While the invention has been described and illustrated with respect to certain alloys of titanium, it is to be understood that the tendency of titanium articles to suffer crevice corrosion may also be reduced by the use of the acid composition method of this invention either where other titanium alloys or unalloyed titanium itself are used. In order that those skilled in the art may more completely understand the present invention and the preferred methods by which the same may be carried out, the following specific examples are offered:

EXAMPLES I THROUGH VIII In each of Examples I through VIII, a 1 inch by 2 inch by 0.008 inch titanium coupon was used for testing. An analysis of the titanium coupons prior to testing showed a nickel content of 1.42 per cent and an iron content of 0.07 weight per cent. Each coupon was then contaminated with iron on one surface. This was done by clamping the coupon in a vise on a drill press. A blunt iron rod was inserted in the drill press bit, the drill press turned on, and the iron rod pressed onto the coupon.

Thereafter, each coupon was inserted into an acid solution at a temperature of 25 to 30C. as described hereinafter. Each coupon was removed from the acid solution after 5 minutes of immersion for the purpose of testing the surface for iron content. If the coupon showed the presence of surface iron, it was reimmersed in the acid solution for 5 minutes and tested again. This was continued until the test for surface iron was negative.

The surface iron content was determined according to the method of ASTM A-380-57, paragraph 7 (c) (1), by the presence or absence of a dark blue color. According to this method on aqueous indicator solution containing 10 grams of potassium ferric cyanide (K Fe(CH) and 30 milliliters of concetrated (70 weight per cent) nitric acid per liter was prepared. Each time a coupon was removed from the acid solution, it was rinsed with distilled water and several drops of indicator solution were applied to the surface. Iron was considered present if the indicator turned dark blue on the surface of the titanium coupon within 30 seconds.

EXAMPLE I A titanium alloy coupon prepared and contaminated with iron as described above was inserted in a 10 volume per cent solution of nitric acid. The coupon was tested every 5 minutes of immersion for the presence of iron for 30 minutes. After 30 minutes there was still sufficient iron contamination present on the surface to turn the indicator dark blue.

EXAMPLE u A titanium alloy coupon prepared and contaminated with iron as described above was inserted in a 10 volume per cent solution of hydrochloric acid. The coupon was removed from the acid after every 5 minutes of immersion and tested for the presence of iron contamination as described hereinabove. After 30 minutes there was still sufficient iron contamination on the surface to turn the indicator dark blue within 30 seconds as described hereinabove.

EXAMPLE III A titanium alloy coupon prepared and contaminated as described hereinabove was inserted in a solution containing 10 volume per cent nitric acid and 10 volume per cent hydrochloric acid. The coupon was removed and tested for the presence of surface iron contamination after every 5 minutes of immersion as described hereinabove. After 30 minutes there was still enough iron contamination on the surface to turn the indicator dark blue within 30 seconds as described hereinabove.

EXAMPLE IV EXAMPLE V A titanium alloy coupon prepared and contaminated with iron as described hereinabove was inserted in a solution containing 10 volume per cent nitric acid and 30 volume per cent hydrochloric acid. After every 5 minutes of immersion the coupon was removed from the solution and tested for the presence of iron as described hereinabove. After 25 minutes there was insufficient iron contamination remaining on the surface of the coupon to change the indicator to dark blue within 30 seconds.

EXAMPLE VI A titanium alloy coupon was prepared and contaminated with iron as described hereinabove. The coupon was inserted in a solution containing 10 volume per cent nitric acid, 1 volume per cent hydrofluoric acid. The coupon was removed from the solution and tested for the presence of surface iron contamination every 5 minutes as described hereinabove. After 25 minutes there was still sufficient surface iron contamination to change the color of the indicator within 30 seconds.

EXAMPLE VII A titanium alloy coupon was prepared and contaminated with iron as described hereinabove. The coupon was inserted in a mixture of 10 volume per cent nitric acid and 5 volume per cent hydrofluoric acid. The coupon was removed and tested for surface iron contamination after every 5 minutes of immersion as described hereinabove. After 20 minutes of immersion in the acid mixture there was no longer sufficient surface iron contamination remaining on the coupon to change the indicator blue within 30 seconds.

EXAMPLE VIII A titanium alloy coupon was prepared and contaminated as described hereinabove and immersed in a solution of 10 volume per cent nitric acid and 10 volume per cent hydrofluoric acid. The coupon was removed and tested for surface iron contamination after every minutes of immersion as described hereinabove. After minutes of immersion in the acid mixture there was insufficient surface iron contamination to change the color of the indicator within 30 seconds. During the immersion, significant gas evolution and loss of titanium was observed.

EXAMPLE IX A titanium alloy coupon was prepared and contaminated with iron as described hereinabove. The coupon was brushed with a wire brush sanding wheel for approximately 5 minutes at normal hand pressure. Thereafter the coupon was washed with distilled water and immersed in a solution of 10 volume per cent nitric acid and 20 volume per cent hydrochloric acid. The coupon was removed after every 5 minutes of immersion to test for iron contamination as described hereinabove. After 5 minutes of immersion there still remained on the surface of the coupon sufficient iron contamination to turn the indicator blue within 30 sec onds. After 10 minutes of total immersion time, there was insufficient iron remaining on the surface of the coupon to turn the indicator blue within 30 seconds.

EXAMPLE X A titanium alloy coupon was prepared and contaminated with iron as described hereinabove. The coupon was sanded with normal 120 220 mesh fine emery cloth at hand pressure for approximately 5 minutes. Vertical and horizontal strokes were applied randomly. Thereafter the surface of the coupon was washed with distilled water and inserted in a solution containing 10 volume per cent nitric acid and 20 volume per cent hydrochloric acid. The coupon was removed from the solution after every 5 minutes of immersion to test iron contamination as described hereinabove. After 5 minutes of total immersion and after 10 minutes of total immersion sufficient iron contamination remained on the surface of the coupon to turn the indicator solution blue within 30 seconds. After minutes of total immersion there was insufficient iron contamination remaining on the surface of the coupon to turn the indicator blue within 30 seconds.

EXAMPLE Xl A titanium alloy coupon is prepared and contaminated as described hereinabove. A solution containing 10 per cent nitric acid and 20 per cent sulfuric acid is prepared. The coupon is inserted in the acid solution. After every 5 minutes of immersion in the solution the coupon is removed from the solution, rinsed with distilled water, and tested for the presence of surface iron contamination. After 30 minutes of total immersion there is insufficient surface iron contamination to turn the indicator blue within 30 seconds.

It is to be understood that although the invention has been described with specific reference to particular embodiments thereof, it is not to be so limited since changes and alterations therein may be made which are within the full intended scope of this invention as defined by the appended claims.

I claim:

1. A method of fabricating an apparatus having titanium structural members for use in a chlorine containing environment comprising the steps of:

assembling the apparatus with iron-containing tools whereby iron inclusions are deposited in the titanium; and

contacting the surface of the titanium with an acid composition comprising an oxidizing acid and a second acid, capable of forming soluble iron salts, whereby the iron inclusions are removed from the titanium.

2. The method of claim 1 wherein the titanium is in an alloy comprising nickel.

3. The method of claim 1 wherein the oxidizing acid is chosen from the group consisting of nitric acid, chromic acid, and perchloric acid.

4. The method of claim 1 wherein the oxidizing acid is I-INO 5. The method of claim 4 wherein the acid composition comprises from about 5 to 15 volume per cent HNO 6. The method of claim 1 wherein the second acid capable of forming a soluble iron salt is chosen from the group consisting of HF, HCl, HI, H SO CCl COOH, and CF COOH.

7. The method of claim 1 wherein the second acid capable of forming a soluble iron salt is I-ICl.

8. The method of claim 7 wherein the acid composition comprises from about 15 to about 40 volume per cent HCl.

9. The method of claim 6 wherein the second acid is HF.

10. The method of claim 6 wherein the second acid is HBr.

11. The method of claim 1 wherein the said acid composition comprises nitric acid and hydrochloric acid.

12. The method of claim 1 wherein the acid composition comprises from about 5 to about 15 volume per cent nitric acid and from about l5 to about 40 volume per cent hydrochloric acid. 

2. The method of claim 1 wherein the titanium is in an alloy comprising nickel.
 3. The method of claim 1 wherein the oxidizing acid is chosen from the group consisting of nitric acid, chromic acid, and perchloric acid.
 4. The method of claim 1 wherein the oxidizing acid is HNO3.
 5. The method of claim 4 wherein the acid composition comprises from about 5 to 15 volume per cent HNO3.
 6. The method of claim 1 wherein the second acid capable of forming a soluble iron salt is chosen from the group consisting of HF, HCl, HI, H2SO4, CCl3COOH, and CF3COOH.
 7. The method of claim 1 wherein the second acid capable of forming a soluble iron salt is HCl.
 8. The method of claim 7 wherein the acid composition comprises from about 15 to about 40 volume per cent HCl.
 9. The method of claim 6 wherein the second acid is HF.
 10. The method of claim 6 wherein the second acid is HBr.
 11. The method of claim 1 wherein the said acid composition comprises nitric acid and hydrochloric acid.
 12. The method of claim 1 wherein the acid composition comprises from about 5 to about 15 volume per cent nitric acid and from about 15 to about 40 volume per cent hydrochloric acid. 